This invention relates to digital video signal reproducing apparatus, such as a digital video tape recorder (VTR) wherein each line of video information is digitized, and each digitized line is separated into separate channels which are recorded, and, more particularly, to apparatus which is capable of reproducing such pluralchannel digitized video signals in special reproducing modes, such as fast search, reverse, stop-motion and the like.
Recently, digital video tape recorders (VTR's) have been developed for recording an analog video signal in digital form. In a typical digital VTR, one or more rotary magnetic heads scan successive, parallel, skewed tracks across the magnetic tape, and digitized video signals are recorded in such tracks. Each line interval of video information is sampled at a relatively high sampling rate on the order of about 3 to 4 times the chrominance subcarrier frequency f.sub.SC ; and these samples are converted to, for example, 8-bit digital words. Each word thus represents a sample of the analog, composite color video signal. Because of the very high recording density which is needed to record such a digitized video signal, it has been suggested that the samples which constitute each line interval be recorded in separate channels, or tracks, on the magnetic tape. To facilitate such separate-channel recording, groups of digitized samples are formed into sub-blocks, each sub-block being constituted by a multiple of words, and then the sub-blocks are recorded in separate, parallel channels by, for example, separate, aligned recording heads which simultaneously scan such tracks. During reproduction of such digitized video signals, playback heads, which may be the very same heads that were used for recording, reproduce the sub-blocks from the respective tracks. Then, after correcting for time-base errors, recording/reproducing errors, and the like, the separate channels of digitized video signals are merged; and the original, analog composite color video signal may be recovered therefrom.
It is desirable to provide such digital VTR's with the same versatility as found in commercially available analog VTR's. Accordingly, a digital VTR desirably should be capable of operating in special reproducing modes in addition to the normal reproducing mode. Typical of such special reproducing modes, as currently available in analog VTR's, includes a fast-search mode, wherein the video tape is driven in the forward direction at a relatively fast speed and a so-called fast-motion video picture is reproduced which enables a viewer to visually scan the recorded video signals until a desired segment of information is reached. In this quick search mode, the video tape normally is driven at a speed on the order of 10 or a multiple of 10 times the normal tape speed which is used for recording and normal reproduction. At such high tape speeds, the playback heads of a digital VTR normally will traverse tracks of different channels. For example, playback head A will traverse tracks A, B, and so on so as to reproduce the digitized video signals, or sub-blocks, from each track which is scanned. This means that, although a particular playback head is associated with a particular channel, and although a respective track has sub-blocks associated only with a predetermined channel recorded therein, that playback head will reproduce sub-blocks from the proper channel as well as sub-blocks from different channels. Since a particular playback head thus picks up data from one or more channels with which it is not associated, such data interferes with the satisfactory recovery of video information. That is, the reproduction of data from channel B by playback head A results in serious degradation of the video information which is reproduced from the magnetic tape. For this reason, the implementation of special reproducing modes in digital VTR's heretofore has been difficult, expensive and complicated.